Diacylglycerol O-Acyltransferase (EC 2.3.1.20), also known as diglyceride acyltransferase or DGAT, is a critical enzyme in triacylglycerol synthesis. Triacylglycerols are quantitatively the most important storage form of energy for eukaryotic cells. DGAT catalyzes the rate-limiting and terminal step in triacylglycerol synthesis using diacylglycerol and fatty acyl CoA as substrates. As such, DGAT plays a fundamental role in the metabolism of cellular diacylglycerol and is important in higher eukaryotes for intestinal fat absorption, lipoprotein assembly, fat storage in adipocytes, milk production and possibly egg production and sperm maturation.
Diacylglycerol is the precursor of such important lipids as triacylglycerol and phospholipids, which store energy and form cellular membranes. In eukaryotes, two major pathways for synthesizing diacylglycerol exist: the glycerol phosphate pathway and the monoacylglycerol pathway. Both pathways generate diacylglycerol that can be used as a substrate by acyl CoA:diacylglycerol acyltransferase (DGAT) for triacylglycerol synthesis. In the glycerol phosphate pathway, which functions in most cells, diacylglycerol is derived by the dephosphorylation of phosphatidic acid produced by sequential acylations of glycerol phosphate. In the monoacylglycerol pathway, which has been reported predominantly in the intestine, diacylglycerol is formed directly from monoacylglycerol and fatty acyl CoA in a reaction catalyzed by monoacylglycerol acyltransferase (MGAT) (E.C. 2.3.1.22).
MGAT is best known for its role in fat absorption in the intestine, where the fatty acids and sn-2-monoacylglycerol generated from the digestion of dietary fat (mainly triacylglycerol) are resynthesized into triacylglycerol in enterocytes for chylomicron synthesis and secretion. MGAT catalyzes the first step of this process, in which fatty acyl CoA, formed from fatty acids and CoA, and sn-2-monoacylglycerol are covalently joined. Because the monoacylglycerol pathway predominates in intestinal triacylglycerol synthesis, MGAT may be a pharmaceutical target for modulating fat absorption.
MGAT activity is also found at high levels in liver of suckling rats and in white adipose tissue of migrating sparrows, where triacylglycerols are actively hydrolyzed to provide fatty acids for energy. MGAT preferentially acylates monoacylglycerols that contain a polyunsaturated fatty acyl moeity at the sn-2 position. Thus, MGAT may preserve essential fatty acids, all of which are polyunsaturated, by resynthesizing them into triacylglycerols. This function may be relevant in mammalian white adipose tissue, which possesses significant levels of MGAT activity. In addition, MGAT may also play a role in signaling, since its product, diacylglycerol, and one of its substrates, 2-arachidonoylglycerol, are signaling molecules.
Like many enzymes that participate in neutral lipid synthesis, MGAT has proven difficult to purify to homogeneity, and an MGAT gene has not been identified. Several partial purifications of MGAT enzymes have been reported, and a 43-kDa MGAT enzyme was purified recently from peanut cotyledons. Difficulties in the purification of MGAT may reflect its hydrophobicity or its involvement in an enzyme complex.
Because of its central role in a variety of different processes, there is much interest in the identification of polynucleotides encoding proteins having DGAT and MGAT activity, as well as the proteins encoded thereby.
Relevant Literature
Of particular interest are: U.S. Pat. No. 6,100,077; and PCT Published Application Nos. WO 98/55631; WO 99/67268; WO 00/01713; WO 99/67403; WO 00/32793; WO 00/32756; WO 00/36114; WO 00/60095; WO 00/66749.
Also of interest are: Smith et al., Nat.Genet. 2000 (25), 87–90). Cases et al., “Identification of a gene encoding an acyl CoA:diacylglycerol acyltransferase, a key enzyme in triacylglycerol synthesis,” Proc. Natl. Acad. Sci. USA (October 1998) 95:13018–13023; Oelkers et al., “Characterization of Two Human Genes Encoding Acyl Coenzyme A: Cholesterol Acyltransferase-Related Enzymes,” J. Biol. Chem. (Oct. 9, 1998) 273:26765–71; and Cases et al. (2001) J. Biol. Chem. 276:38870–38876.
References describing the role DGAT plays in various biological processes include: Bell & Coleman, “Enzymes of Glycerolipid Synthesis in Eukaryotes,” Annu. Rev. Biochem. (1980) 49: 459–487; Lehner & Kuksis, “Biosynthesis of Triacylglycerols,” Prog. Lipid Res. (1996) 35: 169–201; Brindley, Biochemistry of Lipids, Lipoproteins and Membranes (eds. Vance & Vance) (Elsevier, Amsterdam) (1991) pp 171–203; Haagsman & Van Golde, “Synthesis and Secretion of Very Low Density Lipoproteins by Isolated Rat Hepatocytes in Suspension: Role of Diacylglycerol Acyltransferase,” Arch. Biochem. Biophys. (1981) 208:395–402; Coleman & Bell, “Triacylglycerol Synthesis in Isolated Fat Cells. Studies on the Microsomal Diacylglycerol Acyltransferase Activity Using Ethanol-Dispersed Diacylglycerols,” J. Biol. Chem. (1976) 251:4537–4543.
References discussing MGAT activity and purification include: Coleman and Haynes (1984) J. Biol. Chem. 259:8934–8938; Mostafa et al. (1994) Lipids 29:785–791; Xia et al. 1993) Am. J. Physiol. 265:R414–R419; Jamdar et al. (1992) Arch. Biochem. Biophys. 296:419–425; Manganaro et al. (1985) Can. J. Biochem. Cell Biol. 63:341–347; Bhat et al. (1993) Arch. Biochem. Biophys. 300:663–669; Tumaney et al. (2001) J. Biol. Chem. 276:10847–10852; Lehner and Kuksis (1995) J. Biol. Chem. 270:13630–13636.